(1) Field of the Invention
The present invention relates to the driving method for the LCD electrooptic-switching element and the manufacturing of the automatic electronic driving circuitry for the LCD electrooptic-switching element, which is among other applications especially interesting as the basic active element in the various optical systems and automated protection devices, such as automatic welding helmets.
The present invention relates to the driving method and the manufacturing of the automatic electronic driving circuitry for the LCD electrooptic switching element, which allows for the:                long-term autonomous functioning independent of the external power sources        optimizing of the electric driving signals in the sense of the fast electrooptical switching dynamics, long life time of the LCD electrooptic switching elements and the minimal use of energy (several years without changing the batteries):                    multilevel electronic driving of the LCD electrooptic switching elements            minimal consumption of the electric energy for the driving of the LCD electrooptic switching elements            compensation of the DC component of the electric driving signals for the LCD electrooptic switching elements                        
(2) Description of Related Art
So far several technical solutions and applications of the electrooptical switching elements were made. The solutions are disclosed in the following patents: the use of the double “twist-nematic” LCD cell (Gurtler, U.S. Pat. No. 3,890,628); one TN and one “Guest-host” LCD cell Moriyama, U.S. Pat. No. 3,967,881); the use of the LCD or the ceramic active element (Budminger, FR 2,293,188); the LCD with passive and active cell (Hornell, EP 0,005,417); the optical switch and the variable polarizer (Fergason, U.S. Pat. No. 5,074,647). There are also a number of partial solutions of said technical problem. All of the modern technical solution incorporate an autonomous battery power supply supported by the semiconductor solar cell which significantly increases the life time of the battery power supply (Pfanzelt, DE 3,017,241: Bruhin, EP 0,091,514: Tyers, GB 2,138,590: Bruhin CH 671,485: Stanelli, EP 0,331,861):                minimization of the electric energy consumption and the optimization of the electric driving signal is guaranteed either by the use of the automatic turn-off of the protective automatic LCD welding filter and manual turn-on just prior to the welding (Fergason, U.S. Pat. No. 5,377,032), or with the low-frequency driving of the LCD electrooptic switching elements which significantly reduces the consumption of the electric energy in such a way that the automatic turn-off is no longer needed (Gunz, Ghisleni, EP 550,384, U.S. Pat. No. 5,315,099). While the former patent application does not represent any significant novelty and is not solving the problem of the electric power consumption satisfactorily, the later solution is technically very important.        
Solving the problem of the low power consumption by using slowly varying electric driving signals seems to be the best general technical solution at present. However the use of such driving schemes results in several new problems that have not been adequately solved so far:                Synchronization of the driving signals with the time intervals, when the LCD electrooptic switching element has to be in the optically closed state (for example: welding),        Efficient compensation of the long-term DC component of the electric driving signals,        Electric driving field screening due to the ionic conductivity effects in liquid crystals (variations of light attenuation in the optically closed state of the LCD electrooptic switching element,        Electric driving field screening due to the electric potential build-up at the orienting polymer/liquid crystal interface—“residual DC”.In their patents (EP 550,384, U.S. Pat. No. 5,315,099) Gunz and Gisleni manage to solve the above-specified problems only partially: Since it is not possible to synchronize the driving signals at such low frequencies with the time periods when the LCD electrooptical element has to be in the closed state (for example: welding), the above described LCD light shutter and the electric driving technique cannot provide a complete compensation of the DC component of the electric driving voltage, resulting in the reduced life time of the LCD electrooptic switching elements. The authors of the patent application EP 550,384 diminish this problem by changing the phase of the electric driving signals with every activation of the LCD electrooptical element, which only reduces the consequences and does not represent a reliable solution of the problem. As they cannot avoid the adverse effects of the long-term DC component of the electric driving signals, Gunz and Gisleni suggest the use of additional protective layers in the LCD light shutter (U.S. Pat. No. 5,315,099). The gradual build-up of the DC component of the electric driving signals results in pronounced light attenuation variations in the closed state of the LCD electrooptic switching element (for example: welding) due to the screening of the electric field caused by the above mentioned “residual DC” effect. As the result of this the technical solution, as proposed by Gunz and Gisleni, cannot be used with the frequencies above 0.3 Hz (typically), since the electric field screening phenomena become too apparent and distract the user.        
The problems, related to the build-up of the long-term DC component of the electric driving signals, also cannot be adequately solved by the standard long-term DC component compensation techniques, disclosed in the patents (U.S. Pat. No. 4,205,311, JP 08082785) and published in the IBM technical disclosure bulletin 35, 3, Aug. 1992 as well as in SID Digest 20, 226, 1989. All these techniques are based on the formation of a mean DC voltage, connected to one of the two electrodes of the LCD electrooptic-switching element. This “mean DC voltage” is selected in such a way that the long-term DC component of the electric driving signals is eliminated. It is generated by the integration of the electric driving signal over a longer time interval (i.e. large number of the electric driving signal periods). Therefore such a solution is not very applicable in the case of fast, strong transient “DC effects”, which are characteristic for many applications (for example: welding), when using electric driving signals with slowly varying polarity. Due to a slow response of the “mean DC voltage” generating system, it takes quite some time before the DC component, built-up during the transient effect, averages out. Therefore the “residual DC” effects, as described above, can become very pronounced and can distract the user. Furthermore the above-described technique reduces the effective amplitude of the electric driving signal for the LCD electrooptic switching element by a factor of 2. This can be an important obstacle in cases, when high switching speeds are required (for example: welding). Another known solution proposed by Fergason, U.S. Pat. No. 5,347,383, U.S. Pat. No. 5,252,817 uses the dual-frequency driving, the frequency being dependent on the optical state that the electrooptical switching element currently occupies. This allows for the quick changing of the polarity of the electric driving signals when the filter is in the optically open state (reduced flickering of the filter) and slow changing when the filter is optically closed and the flickering is not so pronounced. The consumption of the electric energy is therefore reduced, however only in one optical state, which does not represent the optimal solution of the problem of the consumption of the electric energy.
The increased switching speed of the LCD electrooptic switching elements is generally achieved by using the high amplitude of the electric driving signals (Heimeier, U.S. Pat. No. 3,575,491, U.S. Pat. No. 3,731,986). The optimal results can be achieved by using the appropriate time dependence of the amplitude of the electric driving signals for the LCD electrooptic switching elements (FIG. 1) as disclosed in the patent application (Toth, EP 0,157,744). According to this technical solution the LCD electrooptic switching element is already in the “open state”, driven with the electric signals, the amplitude of which is smaller than the voltage threshold required for the electrooptical switching. The switching speed to the closed state of the LCD electrooptical-switching element is therefore significantly increased. The amplitude of the electric driving signals is very high immediately after the activation and decreases to the voltage level, which is required to maintain the required optical light attenuation.